[Super errors] Add another uncurried arity test

Author: chenglou

jscomp/build_tests/super_errors/expected/arity_mismatch3.re.expected

@@ -0,0 +1,8 @@
+
+ We've found a bug for you!
+ /.../fixtures/arity_mismatch3.re:1:23-35
+ 
+ 1 │ Belt.Array.mapU([||], (. a, b) => 1);
+ 
+ The field Js.Fn.I2 belongs to the record type Js.Fn.arity2
+ but a field was expected belonging to the record type Js.Fn.arity1

jscomp/build_tests/super_errors/fixtures/arity_mismatch3.re

@@ -0,0 +1 @@
+Belt.Array.mapU([||], (. a, b) => 1);